1. Field of the Invention
The present invention relates to a cross-connect device for use in a network for transmitting signals of high-level groups for digital communications, and more particularly to a digital cross-connect system which employs three-stage switches of the basic CLOS type and is capable of branch connections.
2. Description of the Related Art
Cross-connect devices are positioned in the hubs of a transmission network for transmitting signals of high-level groups for digital communications including time-division multiplex communications. In response to commands from a network management system which controls the connections of the entire network, the cross-connect devices switch connections between a plurality of N input signal lines and a plurality of G output signal lines at the respective hubs for thereby changing transmission routes on the network for digital communications. One conventional cross-connect device is basically composed of three switch groups, i.e., former-stage, intermediate-stage, and latter-stage switch groups, constructed by suitably combining a group of space switches (S switches) which spatially switch time slots between a plurality of multiplexed signal series, and a group of time switches (T switches) which switch the chronological sequence of one multiplexed signal series.
With such multistage connections, there may occur a condition referred to as interblocking in which any idle paths between the input signal lines and the output signal lines cannot be located because the intermediate-stage switches are blocked. One proposal to eliminate such a drawback is a multistage switch, called a CLOS-type multistage switch proposed by Charles Clos, in which the number m of intermediate-stage switches is equal to or greater than the sum of the number n of front-stage input signal lines and the number g of rear-stage output signal lines, minus 1 (m.gtoreq.n+g-1) ("A study of Non-Blocking Switching Networks", Bell System Technical Journal, Vol. 32, No. 2, 1953, pp. 406-424).
A CLOS-type three-stage switch for making any desired connections between a plurality of N input signal lines and a plurality of G output signal lines, which is said to have a switch size N.times.G, comprises a primary switch group of N/n switches to which divisions, each composed of n input lines, of N input lines are connected, a tertiary switch group of G/g switches to which divisions, each composed of g output lines, of G output lines are connected, and a secondary switch group of m switches interconnecting the primary and tertiary switch groups, where n and g are integers equal to or smaller than N and G, respectively, and if the values N/n and G/g are not integers, then they represent immediately higher integers. Each of the switches of the primary group has a switch size n.times.m, each of the switches of the secondary group has a switch size (N/n).times.(G/g), and each of the switches of the tertiary group has a switch size m.times.g.
A j-th output terminal of the i-th primary switch is connected to an i-th input terminal of the j-th secondary switch, and a q-th input terminal of the p-th tertiary switch is connected to a p-th output terminal of the q-th secondary switch.
A five-stage switch may be handled as a three-stage switch composed of a primary switch group, a switch block group, and a quintic switch group, the switch block group comprising a CLOS-type three-stage switch group which replaces secondary, tertiary, and quartic switch groups in the intermediate stage. Similarly, switches of more odd-numbered stages may be obtained by replacing intermediate-stage switch groups with a CLOS-type three-stage switch group.
A five-stage switch having a switch size N.times.G comprises a primary switch group of N/n switches having a switch size n.times.m, a quintic switch group of G/g switches having a switch size m.times.g, and a switch block group of m switch blocks having a switch size (N/n).times.(G/g). A j-th output terminal of the i-th primary switch is connected to an i-th input terminal of the j-th secondary switch, and a q-th input terminal of the p-th quintic switch is connected to a p-th output terminal of the q-th switch block.
The intermediate-stage switch block group has an overall switch size N'.times.G' where N'.times.G' is rewritten from (N/n).times.(G/g) and N'=N/n, G'=G/g. Each switch block comprises a secondary switch group of N'/n' switches having a switch size n'.times.m', a tertiary switch group of m' switches having a switch size (N'/n').times.(G'/g'), and a quartic switch group of G'/g' switches having a switch size m'.times.g'. A j'-th output terminal of the i'-th secondary switch is connected to an i'-th input terminal of the j'-th tertiary switch, and a q'-th input terminal of the p'-th quartic switch is connected to a p'-th output terminal of the q'-th tertiary switch.
The input and output signal lines may be connected in either a one-to-one connection configuration in which one input line and one output line are interconnected or a one-to-many branch connection configuration in which one input line is branched and connected to a plurality of output lines. The conventional CLOS-type cross-connect devices operate only in the one-to-one connection configuration, and there have not been available any conventional cross-connect devices operating in the one-to-many branch connection configuration.
The cross-connect devices which operate only in the one-to-one connection configuration may be used principally for circuit switching to select and connect any desired circuit. In addition, they may also be used to switch between transmission routes to keep a substitute transmission path for efficient operation of the network in the event of a failure of a transmission path.
Those of the cross-connect devices operating in the one-to-many branch connection configuration which are capable of one-to-two branch connections may be used as transmission route switching means for branching one signal into two signals at the transmission side of a transmission line, delivering the signals along respective different paths, and selecting one of the signals that has been received under better conditions at the reception side of the transmission line. This process is called patching and rolling, and can shorten the time required for transmission path switching because signals are transmitted simultaneously along two paths.
The cross-connect devices which operate in the one-to-many branch connection configuration may be used in situations where one signal is to be transmitted simultaneously to a number of locations.
The conventional cross-connect devices that employ a multistage switch having a switch size N.times.G are, however, disadvantageous in that accidental blocking cannot be prevented from happening in cases where one input signal is branched into a plurality of k output signals and thereafter branch connections or one-to-one connections are made in one device.